Aluminum alloy brazing sheet

ABSTRACT

A brazing sheet comprises a thin covering material and a core material as well as an Al—Si alloy brazing material inserted between the thin covering material and the core material. The brazing material further contains Mg, Bi and/or Zn, Sn, In. When the above brazing material is molten in an inert gas atmosphere, this molten brazing material seeps onto the surface of the above thin covering material, whereby the brazing sheet is fluxlessly brazed to joint materials.

FIELD OF THE INVENTION

[0001] The present invention relates to an aluminum alloy brazing sheetcapable of fluxless joint in an inert gas atmosphere, a brazing processusing the same, and a brazed product using the same.

BACKGROUND OF THE INVENTION

[0002] An aluminum brazing process has been used for manufa-cturing heatexchangers and panels as it is an efficient method capable of makingjoints with large areas and joints with multiple points. For thesemulti-point joints, a brazing sheet clad with a brazing material as acovering material of a core material which in turn serves as astructural material is generally used by virtue of an arrangementefficiency of the brazing material for the joints.

[0003] Three brazing methods, namely, (1) a fluxless method in vacuum,(2) a non-corrosive flux method in an inert gas atmosphere, and (3) acorrosive flux method in air, have hitherto been used.

[0004] Among these methods, there have been problems because method (1)requires an expensive vacuum furnace, method (2) requires an airtightatmosphere furnace which is cheaper than the vacuum type but relativelyexpensive, a flux coating process, and an expensive inert gas which isconsumed, and method (3) requires a large amount of labor and cost forprocessing a corrosive flux before and after brazing although it isexcellent in apparatus and consumable expense.

[0005] In the light of these problems, for example, a method has beenproposed in which brazing without flux is carried out in a nitrogenatmosphere using a brazing sheet comprised of an Al—Si—Mg—Bi brazingmaterial, as shown in Welding Journal, October 1983, page 31.

[0006] However, this method has been still problematic in that anoxidation film on an original sheet must be controlled to be thin and aflow of nitrogen gas must be increased for prevention of oxidation.

SUMMARY OF THE INVENTION

[0007] It is therefore an object of the present invention to provide analuminum alloy brazing sheet capable of fluxless joint in a simplerinert gas atmosphere, dissolving the problematic issues in the relatedart as described above.

[0008] According to a first aspect of the present invention, there isprovided an aluminum alloy brazing sheet which comprises: a thincovering material; a core material; and an Al—Si alloy brazing materialas an intermediate material interposed between the thin coveringmaterial and the core material, wherein the thin covering material andthe core material comprise aluminum alloys having a solidus temperaturehigher than a liquidus temperature of the brazing material so that themolten brazing material seeps onto a surface of the thin coveringmaterial when the brazing material is molten in an inert gas atmosphere,thereby allowing fluxless brazing.

[0009] This arrangement enables substantial reduction in the productioncost, partly because brazing can be carried out without expensiveinstallations such as a vacuum furnace and an airtight atmospherefurnace, and flux, and partly because the least inert gas is consumed.It also renders the braze-assembling work easy because the brazing sheetcan be placed in a desired position easily. In addition, the inventivearrangement leads to provision of braze-assembled products that exhibitexcellent corrosion resistance.

[0010] Desirably, the Al—Si alloy brazing material further contains oneor two of from 0.1 to 5% (by mass; the same hereinafter) of Mg and from0.01 to 0.5% of Bi.

[0011] The Al—Si alloy brazing material may further contain one, two orthree of from 0.1 to 5% of Zn, from 0.01 to 0.1% of In, and from 0.01 to0.1% of Sn.

[0012] According to a second aspect of the present invention, there isprovided an aluminum alloy brazing sheet brazing process is providedwhich comprises the steps of: providing an aluminum alloy brazing sheetcomprised of a thin covering material, a core material, and an Al—Sialloy brazing material as an intermediate material interposed betweenthe thin covering material and the core material, wherein the thincovering material and the core material comprise aluminum alloys havinga solidus temperature higher than a liquidus temperature of the brazingmaterial, and conducting brazing using the brazing sheet but withoutflux in an inert gas atmosphere.

[0013] According to a third aspect of the present invention, there isprovided a product which is braze-assembled by an aluminum alloy brazingsheet brazing process which comprises the steps of: providing analuminum alloy brazing sheet comprised of a thin covering material, acore material, and an Al—Si alloy brazing material as an intermediatematerial interposed between the thin covering material and the corematerials, wherein the thin covering material and the core materialcomprise aluminum alloys having a solidus temperature higher than aliquidus temperature of the brazing material, and conducting brazingusing the brazing sheet but without flux in an inert gas atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Certain preferred embodiments of the present invention will bedescribed in detail below, by way of example only, with reference to theaccompanying drawings, in which:

[0015]FIGS. 1A to 1C are cross-sectional views showing layerarrangements of aluminum alloy brazing sheets according to the presentinvention, namely, a layer arrangement clad with the brazing material onboth sides, a three-layer arrangement clas with the brazing material onone side, and a four-layer arrangement clad with the brazing material onone side;

[0016]FIGS. 2A and 2B are schematic side views showing mechanisms ormanner of seeping of the brazing material and wet-spreading of moltenbrazing filler; and

[0017]FIG. 3 is a schematic perspective view showing a T-shaped jointpiece used for brazeability test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The following description is merely exemplary in nature and is inno way intended to limit the invention, its application or uses.

[0019] In a preferred embodiment of the present invention, an Al—Sialloy brazing material is used as an intermediate material between athin covering material and a core material.

[0020] Si is an essential alloying element of the aluminum alloy brazingmaterial and has functions by which a melting point of the brazingmaterial is lowered and fluidity of the molten brazing filler isimproved. Its amount to be added is from 5.0 to 15.0%. When it is lessthan 5.0%, lowering of the melting point is insufficient and thusfluidity of the molten brazing filler is not good. When it is more than15.0%, Si primary crystal is generated, and the molten penetratingamount into the core material is increased as the accumulation of themolten brazing filler to a fillet portion is decreased.

[0021] It is acceptable that approximately 0.5% or less of Fe which isan unavoidable impurity commonly contained in aluminum is comprisedsince it does not harmfully affect the properties of the brazingmaterial.

[0022] It is desirable that the Al—Si alloy brazing material furthercontains one or two of from 0.1 to 5% of Mg and from 0.01 to 0.5% of Bi.

[0023] Mg facilitates wet-spreading of the brazing filler in an inertgas atmosphere. In particular, this function caused by the addition ofMg is effective when concentration of oxygen is high in the inert gasatmosphere. The amount of Mg to be added is from 0.1 to 5%. When Mg isless than 0.1%, there is no facilitating effect on wet-spreading of thebrazing filler, and when more than 5%, the facilitating effect issaturated with no economic advantage.

[0024] In vacuum brazing, Mg mostly evaporates and makes littlecontribution to the improvement in strength after brazing. In brazing inan inert gas atmosphere, Mg remains in great amount and thus contributesto the improvement in strength after brazing.

[0025] Bi coexistent with Mg or alone facilitates wet-spreading of thebrazing filler in the inert gas atmosphere. The amount of Bi to be addedis from 0.01 to 5%. When Bi is less than 0.01%, there is no facilitatingeffect on wet-spreading of the brazing filler, and when more than 5%,the facilitating effect is saturated with no economic advantage.

[0026] One, two or three of from 0.1 to 5% of Zn, from 0.01 to 0.1% ofIn, and from 0.01 to 0.1% of Sn are further contained in the brazingmaterial.

[0027] These elements make the brazing material have sacrificialanti-corrosion and improve corrosion resistance of the core material.The amount of these elements to be added is from 0.1 to 5% for Zn andfrom 0.01 to 0.1% for, In and Sn, respectively.

[0028] When each is less than its lower limit, sacrificialanti-corrosion is insufficient, and when more than its upper limit,sacrificial anti-corrosion is saturated with no economic advantage.

[0029] The aluminum alloys having a solidus temperature higher than aliquidus temperature of the brazing material are used for the thincovering material and the core material.

[0030] Any composition of aluminum alloys may be used so long as theabove requirement is satisfied, and the aluminum alloys can be used byselecting standardized alloys, such as JIS A 1070, 1050, 1100, 1200,3003, 3203, 3004, 5005, 5N01, 6951, 6061, 6063, 6N01 and the like, orthese alloys to which various alloy elements are further added, inconsideration of product strength, corrosion resistance and so on asrequired.

[0031] In a brazing process using flux, Mg contents in joint materials(base materials) and core materials are strictly controlled because Mgreacts with the flux. In the present invention, such strict Mg contentcontrol is not necessary with respect to the joint and core materialsbecause no flux is use. This makes it possible to use alloys whichremain strong after brazing.

[0032] It is desirable that the thin cover material contains no Mgbecause, if it contains Mg, in the course of heating for brazing, anoxidation film or layer is likely to grow on a surface of the thincovering material.

[0033] Brazing sheets having a thickness of 0.05 to 2.0 mm are employedherein. When the sheet thickness is less than 0.05 mm, it is difficultto manufacture and brazing efficiency is reduced due to brazing materialshortage. When the thickness is more than 2.0 mm, it is unnecessarilythick and is not economical.

[0034] Brazing sheets having a thickness of 0.05 to 0.2 mm are used asfin materials. Brazing sheets having a thickness of 1.0 to 1.6 mm aresuitable for use as tank materials for radiators. Brazing sheets havinga thickness falling between those thicknesses are suitably used as platematerials for evaporators.

[0035] The clad ratio of the thin covering material (ratio of thethickness of one layer of the thin covering material to the entire sheetthickness) is from 0.1 to 10%, and the clad ratio of the intermediatebrazing material (ratio of the thickness of one layer of theintermediate brazing material to the entire sheet thickness) is from 5to 20%.

[0036] Their reasons are as follows.

[0037] When the clad ratio of the thin covering material is less than0.1%, it is problematic in that control management of the thickness isdifficult and the covering material is peeled off during a rollingprocess, and when more than 10%, seeping can hardly occur when thebrazing filler melts.

[0038] When the clad ratio of the intermediate brazing material is lessthan 5%, brazing efficiency is reduced due to shortage of the brazingmaterial, and when more than 20%, molten erosion of the core materialoccurs due to excessive brazing-filler and it is undesirable.

[0039] Typical example arrangements of the thin covering material,intermediate brazing material, and core material are illustrated FIGS.1A, 1B and 1C.

[0040] The thin covering material 2 and the intermediate brazingmaterial 1 may be laid on both sides of or one side of the core material3. In this instance, the thin covering material 2 and the intermediatebrazing material 1 may be disposed in a set. When both sides are cladwith the brazing materials, five layers including the core material aregiven as shown in FIG. 1A. When one side is clad with the brazingmaterial, three layers including the core material are given as shown inFIG. 1B.

[0041] Further, a sacrificial anodic layer maybe set on the other sidein addition to one side clad with the brazing material to improvecorrosion resistance. In this case, the four-layer structure is given asshown in FIG. 1C.

[0042] Oxygen concentration in the inert gas atmosphere may be set at1000 ppm or less.

[0043] The lower concentration of oxygen is more preferable in terms ofbrazing efficiency, but the cost is increased since a large quantity ofinert gas is required for attaining 30 ppm or less of oxygenconcentration.

[0044] As described above, when the Al—Si alloy brazing material furthercontains one or two of from 0.1 to 5% of Mg and from 0.01 to 0.5% of Bi,brazing is possible at up to about 1000 ppm of oxygen concentration,whereas it does not contain one or two of from 0.1 to 5% of Mg and from0.01 to 0.5% of Bi, brazing is possible only at up to 500 ppm of oxygenconcentration.

[0045] Nitrogen is usually preferable as the inert gas in terms of cost,but noble gas such as Ar or the like may be used.

[0046] The heating temperature for aluminum sheet brazing should be oneat which the brazing material melts but the covering material alone doesnot, namely, a temperature higher than a liquidus temperature of thebrazing material but lower than a solidus temperature of the thiscovering material, normally in a range of 560 to 620° C.

[0047] The reasons why fluxless brazing is possible under the inert gasatmosphere in the brazing process according to the invention are assumedto be as follows.

[0048] Generally, for enabling aluminum brazing, it is an essentialrequirement to generate wetting of the molten brazing filler. To satisfythis requirement, the functions of anti-oxidation and oxidation filmbreaking are needed in the brazing material and a joint companion (basematerial) during heating for brazing. In this case, it is believednecessary that sensitivity of the both functions is higher especially inthe brazing material and that control is stricter on the brazingmaterial than on the joint companion (base material).

[0049] Thus, of the currently available brazing methods as describedabove, Mg evaporation phenomenon in conjugation with heating in vacuumis utilized (vacuum brazing) or flux in a non-oxidative atmosphere isutilized (non-corrosive flux brazing method in a non-oxidativeatmosphere) for these anti-oxidation and oxidation film breaking.

[0050] On the contrary, in the present invention, as shown in FIG. 2A,the brazing material 1 in the intermediate layer is molten as thetemperature is elevated during the brazing, but oxidation of the surfaceof the brazing material does not occur because the surface is coveredwith the thin covering material 2 which remains solid.

[0051] When the temperature is further elevated, the portions with lowermelting points, such as the segregation portion of the thin coveringmaterial 2 close to the molten brazing material 1, are locally molten,and then the brazing material 1 seeps and spreads over the surface ofthe thin covering material due to volumetric expansion as shown byarrows. In this case, the surface of the brazing material becomes anemerging face without an oxidation film, and new intensive oxidationdoes not proceed due to the inert gas atmosphere.

[0052] The conventional flux used for the brazing in a air or an inertgas atmosphere and Mg for the vacuum brazing are originally used tofacilitate wet-spreading of the molten brazing material by breaking thethick oxidation film which has been generated on the sheet surface or isgenerated during heating for the brazing. However, because no thickoxidation film exists in the invention, the molten brazing materialwettingly spreads even in the absence of flux and Mg.

[0053] When the temperature is further elevated, as shown in FIG. 2B,melting further proceeds such that many portions of the thin coveringmaterial 2 are molten, causing seeping to grow in magnitude. The seepedmolten brazing material 1 continuously wet-spreads over the uppersurface of the thin covering material. At this time, the thin coveringmaterial 2 below the oxidation film also melts in substantial quantityin such a manner as to lose its thickness due to all-out erosion by themolten brazing material. Owing to the molten brazing material spreadover the upper surface of the oxidation film on the surface of the thincovering material and the molten thin covering material advanced frombelow the oxidation film, the oxidation film 4 originally present on thesurface of the thin covering material 2 is dispersed in a harmoniousmixture of the molten brazing material and the molten thin coveringmaterial, resulting in no adverse effect of the oxidation film.

[0054] If appropriate amounts of Mg and Bi present in the brazingmaterial, they consume oxygen in a proximal atmosphere and preventoxidation of the surface of the molten brazing material and the thincovering material. Therefore, wet-spreading of the brazing isfacilitated and a good brazing property is maintained even at a littlehigher concentration of oxygen in the inert gas atmosphere.

DESCRIPTION OF EXPERIMENTS

[0055] The core material was made by scalping of both sides to 40 mm inthickness of the book mold casting ingot of the alloy for core material(No. A) shown in the following Table 1, followed by homogenizingtreatment at 600° C. for 10 hours.

[0056] The thin covering material 1.9 mm thick (clad ratio: 3%) and theintermediate brazing material 9.4 mm thick (clad ratio: 15%) were madeby scalping of both sides to 40 mm in thickness of book mold castingingots of alloys for thin covering material (No. B) and for intermediatebrazing material (No. from C to M), respectively, and then by hotrolling and cold rolling sequentially.

[0057] Then, the thin covering material, intermediate brazing material,and core material were put on by the combination shown in Table 2, werehot-rolled at 500° C. of a starting temperature to be a clad material 4mm thick, and then this was cold-rolled to 0.5 mm in thickness followedby annealing at 400° C. for 2 hours to make 0 tempered material.

[0058] As shown in FIG. 3, a T-type joint test piece was assembled bycombining the brazing sheet 5 with five or three layers (0.5 mm inthickness×30 mm×60 mm) made in this way with JIS A 3003 joint partnermaterial (base material) sheet 6 (1.0 mm in thickness×30 mm×60 mm).TABLE 1 Solidus Liquidus No. Si Fe Mn Mg Bi Zn Sn In Al Temp Temp A —0.3− 1.1 — — — — — Balance 643 654 B 0.1 0.2 — — — — — — Balance 647 658C 10.0 0.2 — — — — — — Balance 577 590 D 10.0 0.2 — 1.5 — — — Balance559 591 E 10.. 0.2 — — 0.2 — — — Balance 577 590 F 10.0 0.2 — 1.5 0.1 —— — Balance 559 591 G 10.0 0.2 — — — 1.0 — — Balance 576 588 H 10.0 0.2— — — — 0.08 — Balance 577 590 I 10.0 0.2 — — — 0.2 — 0.08 Balance 577590 J 10.0 0.2 — 1.5 — 1.0 — — Balance 556 577 K 10.0 0.2 — 1.5 — 2.0 —— Balance 554 575 L 12.0 0.2 — 1.5 — — — 0.05 Balance 577 580 M 10.0 0.2— 1.5 — 0.5 0.03 — Balance 558 590

[0059] TABLE 2 Thin Thin covering Brazing Core Brazing covering No.material material material material material Invention 1 B C A C BExamples 2 B D A D B 3 B E A E B 4 B F A F B 5 B G A G B 6 B H A H B 7 BI A I B 8 B J A J B 9 B K A K B 10 B L A L B 11 B M A M B Comparative 12— C A C — Examples 13 — D A D —

[0060] This test piece was brazed in a furnace (temperature at 600° C.)under a nitrogen atmosphere with various concentrations of oxygen, andtaken out from the furnace 5 min after the maximum temperature wasreached.

[0061] For comparison, the test piece was made as in the above caseexcept that, instead of the covering material of the invention, twokinds of conventional double-side clad brazing sheets were used for thesurface of the brazing material, and it was brazed without flux coatingas in the case of the invention.

[0062] The results of brazing efficiency were evaluated by formationratios of fillets, i.e., a ratio of the length of fillet formation tothe length of brazing joint (the entire length of fillet formation:100%), and are shown in Table 3 below. TABLE 3 Fillet formation ratio(%) Oxygen Oxygen Oxygen Oxygen Oxygen level level level level level1200 No. 50 ppm 100 ppm 500 ppm 800 ppm ppm Invention 1 100 100 100 10 0Examples 2 100 100 100 100 20 3 100 100 100 100 15 4 100 100 100 100 805 100 100 100 15 0 6 100 100 100 10 0 7 100 100 100 10 0 8 100 100 100100 30 9 100 100 100 100 45 10 100 100 100 100 50 11 100 100 100 100 40Comparative 12 0 0 0 0 0 Examples 13 60 40 0 0 0

[0063] From the results of Table 3, the ratios of fillet formation inComparative Examples 12 and 13 without surface thin covering materialsare low, and particularly no fillet is formed in Comparative Example 12in which Mg was not contained in the brazing material. Among Examples,in Examples 1, 5, 6 and 7 wherein the brazing material was neither Mgnor Bi, it was confirmed that good joint was exhibited by all 100% offillet formation ratios when the oxygen concentration is within 500 ppmin the brazing furnace under nitrogen atmosphere, whereas the filletformation ratios are reduced when the oxygen concentration is above 500ppm. On the other hand, among the invention Examples, in Example 2, 3,4, 8, 9, 10, and 11 wherein the brazing material contained either Mg orBi at an appropriate amount, good joint is exhibited by 100% of filletformation ratios even when the oxygen concentration is above 500 ppm inthe brazing furnace under nitrogen atmosphere.

[0064] For evaluation of the corrosion resistance, one month CASS testaccording to JIS H 8681 was carried out using a single sheet afterheating to braze at 50 ppm of oxygen concentration as an externalcorrosion resistant evaluation using the brazing sheets (0.5 mmt×70mm×150 mm) with five layers or three layers. This is the test toevaluate sacrificial anti-corrosion effect for the core material.

[0065] After completion of the test, corrosion products were removed andthe depth of pitting corrosion was determined by a focal depth method.The results are shown in Table 4 below. TABLE 4 Results of CASS testsMaximum depth of Average depth of pitting corrosion pitting corrosionNo. (mm) (mm) Inventive 1 0.26 0.22 Examples 2 0.32 0.26 3 0.30 0.25 40.32 0.26 5 0.13 0.10 6 0.12 0.10 7 0.10 0.08 8 0.18 0.14 9 0.13 0.11 100.14 0.12 11 0.13 0.11 Comparative 12 — — Examples 13 0.38 0.29

[0066] From the results of Table 4, among the brazing sheets of theinvention, it was confirmed that a sacrificial anti-corrosion effectworked and the depths of pitting corrosion was shallow since Zn, Sn, Inwere contained especially in Example 5, 6, 7, 8, 9, 10, and 11corresponds to claim 3.

[0067] In the Examples corresponding to claim 3, as the content of Zn,Sn, and In are increased, the depth of pitting corrosion is prone to beshallower provided that the content of Si and Mg is constant in thebrazing material.

[0068] The present disclosure relates to the subject matters of JapanesePatent Application No.2000-125114, filed Apr. 26, 2000, and JapanesePatent Application No.2001-324321, filed Oct. 23, 2001, the disclosuresof which are incorporated herein by reference in their entireties.

What is claimed is:
 1. An aluminum alloy brazing sheet comprising: athin covering material; a core material; and an Al—Si alloy brazingmaterial as an intermediate material interposed between said thincovering material and said core material, said thin covering materialand said core material being comprised of aluminum alloys having asolidus temperature higher than a liquidus temperature of said brazingmaterial so that the molten brazing material seeps onto a surface ofsaid thin covering material when said brazing material is molten in aninert gas atmosphere, thereby allowing fluxless brazing.
 2. An aluminumalloy brazing sheet according to claim 1, wherein said Al—Si alloybrazing material further contains one or two of from 0.1 to 5% (by mass;the same hereinafter) of Mg, and from 0.01 to 0.5% of Bi.
 3. An aluminumalloy brazing sheet according to claim 1 or 2, wherein the Al—Si alloybrazing material further contains one, two or three of from 0.1 to 5% ofZn, from 0.01 to 0.1% of In, and from 0.01 to 0.1% of Sn.
 4. A brazingprocess using an aluminum alloy brazing sheet, comprising the steps of:providing an aluminum alloy brazing sheet comprised of a thin coveringmaterial, a core material, and an Al—Si alloy brazing material as anintermediate material interposed between said thin covering material andsaid core material, said thin covering material and said core materialcomprising aluminum alloys having a solidus temperature higher than aliquidus temperature of said brazing material; and conducting fluxlessbrazing of said brazing sheet in an inert gas atmosphere.
 5. A productbraze-assembled by a brazing process using an aluminum alloy brazingsheet, the process comprising the steps of: providing an aluminum alloybrazing sheet comprised of a thin covering material, a core material,and an Al—Si alloy brazing material as an intermediate materialinterposed between said thin covering material and said core material,said thin covering material and said core material comprising aluminumalloys having a solidus temperature higher than a liquidus temperatureof said brazing material, and conducting fluxless brazing of saidbrazing sheet in an inert gas atmosphere.